1. Field of the Invention
This invention relates to induction hardening of metals and more particularly to a method for hardening the surface of valve seat inserts by induction hardening and conduction quenching.
2. Description of the Prior Art.
In the manufacture of engine blocks for internal combustion engines it is common practice to heat inductively the generally conical valve seats formed integrally with the block. Thereafter, the heated valve seats are quenched to increase the hardness of the seats to reduce wear that results as poppet valves are operated against the valve seats. Generally, this induction heating process is accomplished by positioning a circular inductor directly opposite the conical surface of the valve seat and energizing the inductor by a high frequency electrical power source.
The valve seat for the exhaust valve in the engine head of an internal combustion engine must have an extremely high wear characteristic at high temperatures. This requirement is becoming more important in engines that use lead free gasoline because the absence of lead increases the amount of wear experienced by the valve seat and valve during operation. The pressure of lead compounds in the gasoline produces a lubricating effect on the surface of the valve seat. The problem is complicated further in engines having an aluminum head; aluminum is more susceptable of wear than is cast iron because it is softer. The hardening process in such engines must therefore be controlled to a closer tolerance of hardness than heretofore required with gray cast iron engine blocks and heads.
The need for a close control of the valve seat hardening process has been recognized in the automobile industry for some time. Aluminum head engines typically have valve seat inserts formed from a high strength and hardness material, for example, powdered or sintered formulations, either cast in place during the engine head casting process or installed in the aluminum head during a subsequent operation. When such inserts are used they must be machined while in place in the engine head to produce a close dimensional tolerance seat for the valve which will assure concentricity with the valve and the push rods. Because machining is required, the inserts are installed in the engine head in a metallurgically soft or annealed condition so that forming tools used during machining will wear at an acceptable rate, the machining operation can be done quickly and the insert will remain fixed in place in the engine head.
Normally, valve seat inserts are installed in the engine head by machining a bore in the head having a diameter that is 0.003-0.005 inches less than the outside diameter of the valve seat inserts. It has been found that by chilling the valve seat inserts to cryogenic temperatures before installation the inserts will be retained in the engine head during the machining operation and during the service life of the vehicle. However, when hardened inserts are installed by shrink fitting, subsequent machining was found to reduce the retention effect of the shrink fit. Consequently, the valve insert becomes prone to disengagement from the bore in the head either during machining or during an unacceptably short portion of the service life of the vehicle.
It has long been recognized in the art of induction hardening that the use of electrical energy in the radio frequency range will limit the depth to which the heat will penetrate the article to be hardened. For example, Brown (U.S. Pat. No. 2,424,794) has used electrical energy at radio frequencies as a source of heat and applies the heat to the surface of the work to be hardened by electromagnetic induction. The patent of Jordan (U.S. Pat. No. 2,444,259) describes a method for hardening irregularly shaped articles, particularly gear teeth, wherein a current having a frequency of 200 Khz is used to heat the tip portions of the gear teeth. It was recognized that by varying the frequency of the current various portions of the gear teeth can be heated selectively. Lower frequencies, for example, were found to be effective in heating the root areas of the teeth. The patent of Edwards (U.S. Pat. No. 2,757,268) describes a method for heating valve seats with the use of an electrical current whose frequency is on the order of 500 KHz. The patent of Lihl (U.S. Pat. No. 3,240,639) describes induction heating processes to obtain an improved microstructure for ferro-carbon alloys. Lihl understood that for a given time of exposure the choice of frequency may be used to control the amount of heat applied and the depth of penetration of the heat into the part to be hardened.
Whenever a ferritic material is hardened, an austenitic grain structure must appear before it is subsequently quenched to form the martensitic structure, the degree of whose presence is a function of the rate at which the material is quenched and the temperature to which the material is heated.
The concept of quenching valve seats by conducting heat away from the heated surface into the interior regions of the engine head rather than by a liquid quench is known in the art. Brown in U.S. Pat. No. 2,424,794 taught that the extreme heat developed during induction heating had to penetrate into the workpiece only a very short distance from the outer, heated surface. Instead of applying air, water or oil for quenching the heated material convectively, the heat was dissipated by conduction to the interior of the part being heated. This procedure for conduction quenching is taught also from U.S. Pat. Nos. 3,737,612 and 3,837,934.
Where the inductive heating process is applied to the surface to be hardened for a long period, distortion of the engine head is a recurring problem. Where the engine heads are formed of gray cast iron this distortion occurs to a particularly unacceptable degree.
When iron exhausts valve seats are press fitted, shrink-fitted or cast-in-place in an aluminum engine head there is a tendency for a later induction hardening process applied to the valve seats to reduce or to overcome the forces tending to retain the valve seat in the head. Subsequent machining operations of the hardened valve seats that produce concentricity and a surface complementary to the outer surface of the valve seat operate further to break the bond between the valve seat and the engine head. In operation, also, the extremely high temperatures of the exhaust gas passing at high velocity over the surface of the valve seat and engine head can cause the valve seat retention forces to be overcome if the engine head expands sufficiently due to thermal effects unless the retention forces between the head and the seat at room temperature are sufficiently high.